Audio equipment used for in-vehicle audio (car audio), home audio, portable audio and the like as a function of changing the frequency characteristic of the audio signal. For example, in a car audio product (or AV amplifier device), a user can select one from a plurality of equalizer settings corresponding to audio genres and musical instruments such as flat, rock, pops, vocals, piano, jazz, classical, etc. (preset equalizer) according to the user's preference. Alternatively, some car audio products have a function of reproducing spaces such as concert halls, jazz clubs, churches and the like (acoustic adjustment function). As another alternative, some car audio products have a function of correcting the frequency characteristic and phase characteristic of an audio signal reproduced from a speaker in accordance with a room sound field so as to obtain the optimal frequency characteristic and phase characteristic at a viewing position (also referred to as fixed equalizer or room correction). These are collectively referred to as frequency correction in this specification.
When such frequency correction is implemented by an analog circuit, the circuit scale becomes enormous. Therefore, a digital signal processor (DSP) is used to perform digital signal processing on a digital audio signal. FIG. 1 is a block diagram of an audio system including a DSP. The audio system 100 includes sound source 102, an analog amplifier 104, an A/D converter 106, a microcomputer 108, a DSP 110, a D/A converter 112, a volume circuit 114, a power amplifier 116 and an electroacoustic transducer 118.
The sound source 102 is a CD player, a DVD player, a silicon audio player, a smart phone or the like and outputs an analog audio signal. The analog amplifier 104 amplifies the analog audio signal from the sound source 102 and matches the amplified analog audio signal with an input range of the A/D converter 106 at the subsequent stage. Alternatively, a digital audio signal generated by the sound source 102 may be directly input to the DSP 110.
The DSP 110 receives a digital audio signal from the A/D converter 106 or the sound source 102 and performs a predetermined digital signal processing on the received digital audio signal. Examples of the signal processing by the DSP 110 may include stereo-monaural conversion, digital volume control and the like in addition to the above-described processing related to frequency correction.
The D/A converter 112 converts the digital audio signal processed by the DSP 110 into an analog audio signal. The volume circuit 114 amplifies an output signal of the D/A converter 112 with a gain corresponding to a volume value. The power amplifier 116 amplifies an output of the volume circuit 114 and drives the electroacoustic transducer 118 such as a speaker or a headphone.
The microcomputer 108 integrally controls the audio system 100. The microcomputer 108 receives a command input by a user through a user interface such as a volume button or a touch panel, and controls the DSP 110 based on the command. The microcomputer 108 and the DSP 110 are interconnected via a bus or a control line. For example, upon detecting a volume change instruction input by the user, the microcomputer 108 transmits a command value of the gain of a digital volume circuit of the DSP 110 to the DSP 110.
FIGS. 2A and 2B are block diagrams related to the frequency correction of the DSP 110. As shown in FIG. 2A, the DSP 110 includes a multi-band equalizer 120 and an interface circuit 130 with the microcomputer 108. The multi-band equalizer 120 includes a plurality of M filters 122_1 to 122_M corresponding to the number of bands M. Upon detecting an equalizer change instruction input by the user, the microcomputer 108 transmits setting data of the filters 122_1 to 122_M.
FIG. 2B is a circuit diagram of the filter 122. This filter 122 is a second-order IIR (Infinite Impulse Response) filter, and its Q value, frequency f and gain can be set by a combination of five coefficients b0 to b2, a0 and a1.
The interface circuit 130 is, for example, an I2C (Inter IC) serial interface and can transmit data with 8 bits as one word. In a case where each coefficient is 24 bits (or 32 bits), in order to transmit five coefficients, it is necessary to transmit data of 24×⅝=15 times per band and, for M=13 bands, data transmission of 13×15=195 times is required.
Various data are always transmitted/received between the microcomputer 108 and the interface circuit 130. As one example, the audio system 100 has a function (spectrum analyzer function) for visually displaying the frequency spectrum of an audio signal being reproduced, and, during the reproduction of the audio signal, the microcomputer 108 reads out the signal strength of each band via the interface circuit 130 of the DSP 110. When data transmission for changing the setting of the equalizer is interrupted, data transmission for the spectrum analyzer is stopped each time, so that the correct frequency spectrum is not displayed. Particularly with the I2C interface, it is necessary to wait for an acknowledgment ACK after sending data from a master to a slave, which may cause an overhead.